The present invention relates to a polarizer and a retardation plate for use in a liquid crystal display device.
Liquid crystal displays have found an expanding range of applications because of high display quality and reduced thickness and power consumption, and they are applied to a variety of products ranging from portable monitors such as displays for mobile phones and displays for digital still cameras to monitors for desktop personal computers, monitors for printing and design, monitors for medical use, and further, liquid crystal televisions.
As the range of applications of liquid crystal displays has been expanded in recent years, still further improvement of quality of the liquid crystal display has been desired, and above all, the needs for a wider view angle, a higher contrast ratio and reduced power consumption have intensified.
The display principle of the liquid crystal display is to display variations of light and shade using the refractive index anisotropy of liquid crystal molecules and the polarization and transmission characteristic of a polarizing plate. However, the refractive index of liquid crystal molecules varies depending on the direction, and accordingly, the refractive index of a liquid crystal layer varies depending on whether the liquid crystal display is viewed from the front or aslant. This is a main factor of causing a change in the color tone and a change in the contrast ratio with the view angle of the liquid crystal display. The black of the liquid crystal display is displayed by crossed nicols of the polarizing plate, but when viewed aslant, an absorption axis of the polarizing plate shifts from orthogonality, resulting in light leakage. Thus, the contrast ratio decreases when viewed aslant. For correction of the problem of the view angle characteristic of the liquid crystal display, a retardation plate capable of optical compensation is used in the liquid crystal display.
General retardation plates include those made by stretching a polymer film and those using a liquid crystal. In the case of a stretched polymer film, an optical axis extends in parallel or perpendicularly to the stretching direction, and therefore when a retardation film is prepared, the optical axis of the retardation film is parallel or perpendicular to the longer direction of the film. The film is cut in accordance with the size of a liquid crystal display element and laminated to the polarizing plate and the liquid crystal display, but a shift of the optical axis occurs at the time of cutting or laminating the film. Furthermore, in the case where the film is cut at a certain angle with respect to the longer direction of the film, or the like, it is difficult to cut the film with high axis accuracy and usage efficiency is poor.
Next, the retardation plate using a liquid crystal requires an alignment film and an alignment treatment thereof. The alignment treatments include a rubbing alignment treatment and an optical alignment treatment. The rubbing alignment treatment has the problem of generation of static electricity and dusts at the time of rubbing and occurrence of unevenness and the like at during a large screen treatment. The optical alignment treatment is useful without generating static electricity or dusts, but has the problem of polarization axis accuracy of irradiated light. In addition, there is the problem of axis accuracy at the time of cutting or lamination to the polarizing plate as in the case of the stretched film type.
The retardation plate is arranged such that the refractive index anisotropy axis of the retardation plate forms a certain angle with the absorption axis of the polarizing plate and the refractive index anisotropy axis of the liquid crystal, and generally, the retardation plate is laminated to the polarizing plate and the liquid crystal panel using an adhesive. At this time, if the optical axis of the retardation plate shifts even slightly, desired optical properties cannot be obtained, and there arises the problem of a change in the color tone and a decrease in the contrast ratio in the liquid crystal display.
As the polarizing plate used in the liquid crystal display device, iodine-based polarizing plates made by impregnating a polyvinyl alcohol with iodine and stretching the resulting material are currently widely used. The iodine-based polarizing plate absorbs one of polarizing components of light and transmits the polarizing component in the orthogonal direction. Such an absorption polarizing plate has the problem of poor light usage efficiency, since it can use at most 50% of the amount of light from a light source of the liquid crystal display device. Patent Document 1 shown below discloses a wire grid polarizing optical element intended for improvement of usage efficiency of light.    Patent Document 1: JP-A-2001-74935
When an absorption polarizing plate such as an iodine-based polarizing plate is used in the liquid crystal display device, usage efficiency of light is 50% or less in principle. On the other hand, a wire grid polarizer is a reflection polarizer reflecting light having an electric vector parallel to the longer direction of a wire (metal wire) and transmitting light having an electric vector in the vertical direction. By reusing light of the reflected component, usage efficiency of light can be improved.
However, the wire grid polarizer has the following three problems. First, the surface of the metal wire of the wire grid polarizer having a metal wire formed on a transparent substrate has low contact resistance, and the metal wire is thus fragile. This raises a problem in handling during manufacture and reliability of the product. Second, when the retardation plate and the polarizing plate are laminated together, an adhesive is required, but the adhesive layer reduces the degree of polarization of straight polarized light which has passed through the polarizing plate, and the contrast ratio of the liquid crystal display device thus decreases. This is ascribable to light scattering caused by the adhesive layer. Third, when the wire grid polarizer is used in the liquid crystal display device, the polarizer is warped by heat of the light source.